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Jul 5, 2010

Carbon scrubbing: in it for the long haul?

It sounds like the perfect solution. Why not just use technology to remove all the carbon dioxide in the atmosphere that man has added since the industrial revolution? Climate change should surely disappear in one fell swoop.

Unfortunately it's not that simple. In the first peer-reviewed study of the response of the climate system to such action, researchers from the Carnegie Institution, US, found that taking out all the anthropogenic carbon dioxide from the atmosphere in 2050 would cut temperatures by just 0.8°C – less than one half of the temperature increase caused by the gas in the first place.

"This finding is significant because it indicates that the percentage of warming reduced will be much less than the percentage of one-time anthropogenic carbon dioxide removal from the atmosphere," said Long Cao. "To maintain global temperature at a low and 'safe' level a long-term project committed to continuous removal of anthropogenic carbon dioxide is needed."

Cao and colleague Ken Caldeira used a coupled climate-carbon cycle model (the University of Victoria Earth System Climate model) to look at the global response to removal of carbon dioxide in 2050. They assumed that atmospheric concentrations had reached 511 ppm under an A2 emission scenario, causing a temperature rise at the surface of 1.8 ° above pre-industrial levels.

At the same time as the removal of atmospheric carbon dioxide from the model, additional emissions were halted. The changes reduced surface air temperature by 0.8 °C within five years, but a warming of one degree above pre-industrial levels remained for several centuries.

Taking carbon dioxide out of the atmosphere encourages increased emissions of the gas from the oceans and soil. (Although a reduction in atmospheric carbon dioxide decreases the amount of soil respiration, which is a source of the gas, it has a much larger effect on the net primary production of plants, decreasing their absorption of carbon dioxide). In the model, the reduction of atmospheric carbon dioxide concentrations to the pre-industrial level of 278 ppm was followed by a rise back up to 362 ppm 30 years later. At this point the oceans began to re-absorb some of the gas released by the terrestrial biosphere, leading to a level of 341 ppm by 2500. In total, just over one-quarter of the carbon dioxide removed was replaced.

"This has obvious implications for the public and for policy makers as we weigh the costs and benefits of different ways of mitigating climate change," said Ken Caldeira. ""If we do someday decide that we need to remove carbon dioxide from the atmosphere to avoid a climate crisis, we might find ourselves committed to carbon dioxide removal for a long, long time. A more prudent plan might involve preventing carbon dioxide emissions now rather than trying to clean up the atmosphere later."

Cao and Caldeira calculated that removing 100 gigatons of carbon dioxide from the atmosphere would cool temperatures by roughly 0.16 °C.

The team also simulated the removal of carbon dioxide in 2050 followed by continual extraction of the carbon dioxide released from the oceans and land as a result. This approach reduced surface air temperature to year-2000 levels within five years, and to 0.1 ° above pre-industrial levels within 70 years. It required the removal of 494 Gt carbon in 2050 and roughly the same amount again over the next 80 years.

If emissions were halted in 2050 without any carbon removal, warming would continue for around a century before temperatures began to decline at about 0.02 deg;/century, according to the researchers.

"Various approaches have been proposed to remove anthropogenic carbon dioxide from the atmosphere, including directly capturing carbon dioxide from ambient air through industrial processes," said Cao.

David Keith is developing "carbon castles" to reduce carbon dioxide levels, while others have pushed for land-use management and enhanced weathering to carry out the job.

"We have looked at the response of the global carbon cycle and temperature to an extreme and idealized scenario of removing all anthropogenic carbon dioxide from the atmosphere," said Cao. "We will continue this study by looking at multiple aspects of the climate-system response, including the response of temperature and the hydrological cycle, to different scenarios of atmospheric carbon dioxide mitigation."